1. Field of the Invention
The present invention relates to facsimile terminal equipment units and particularly, to facsimile terminal equipment units for carrying out a facsimile communication through an ISDN (Integrated Service Digital Network).
2. Description of the Related Art
When facsimile communication is carried out in a circuit switching mode, which may operate using one of several communication speeds unless the same communication speed is established between communication terminals, such a communication is not possible.
Facsimile terminal equipment units that supports a so-called G4 facsimile communication standard have two communication modes, i.e., the circuit switching mode and a packet switching mode. Communication in the circuit switching mode is carried out through, for example, an ISDN circuit or an INS (a kind of ISDN) circuit.
There are typically two communication channels for transmitting signals in the circuit switching mode: the D-channel and the B-channel. Both channels are used when transmitting facsimile data. The D-channel is used for setting up the communication, and the B-channel is used for the transfer of data.
The communication speed of the D-channel of the ISDN circuit, which is standardized throughout the world is 16 Kilo bits per second (Kbps). The communication speed of the B-channel of the ISDN circuit, however, is not standardized, it is 64 Kbps in Japan and Europe, and 56 Kbps in North America.
To maintain communication between G4 facsimile terminals in Japan in the circuit switching mode, it is not necessary to adjust communication speed, whereas between Japan and North America, speed be adjusted due to the use of different communication speeds on the B-channel.
The speed adjustment is performed manually solely through an operator because it is impossible to know the communication speed of an opposing terminal. Namely, an originating facsimile terminal cannot identify the communication speed of the opposite facsimile terminal. To call the U.S. from Japan, for example, the operator enters the number of a facsimile terminal to be called in the U.S. as well as the U.S. communication speed of 56 Kbps. Thereafter, the communication speed is negotiated in a setting up controlling process for the D-channel.
According to this conventional technique, communication between Japan and North America, for example, is performed effectively if the operator enters the correct communication speed. To establish the correct communication speed, the operator must confirm the country of the opposite terminal and the communication speed of the country. This is troublesome for the operator. Since it is impossible to know the communication speed of the opposite terminal from the phone number of the facsimile terminal which the operator is calling it is possible that the operator will enter an incorrect communication speed in calling the opposite terminal.
As an example of such erroneous calling, FIG. 11 shows an originating or transmitting terminal having a communication speed of 64 Kbps calling an opposite or receiving terminal of 56 Kbps. In this example, the originating terminal is not provided with the information that the communication speed of the opposite terminal is 56 Kbps and therefore, has used an incorrect communication speed.
The actual process the two facsimile terminals perform when the incorrect communication speed is used is as follows: Since communication speed on the D-channel of ISDN is identical for every terminal connected to the ISDN, a setting up process (Steps i and ii) is done on the D-channel, and the opposite terminal returns acknowledgement signal (Step iii). As a result, the originating and received terminals then initially are then initially connected to each other through the B-channel of the ISDN (Step iv), and the originating terminal tentatively transmits a flag pattern at 64 Kbps on the B-channel (Step v-1). The opposite terminal cannot identify the flag pattern because of the difference in communication speed, and therefore, sends no acknowledging flag pattern. The originating terminal repeatedly transmits the flag pattern for a predetermined time period (for example, 20 to 60 seconds or more) (Steps v-1 through v-n). The originating terminal, since no reply is received, disconnects the B-channel without communicating facsimile data (Steps viii through xiii).
A similar matter may occur if data for negotiating communication speed is abandoned in the middle of relaying the data on the B-channel through many switching systems, or if a circuit failure occurs. The switching systems will also be unable to identify the appropriate communication speed to reestablish communication.
An additional problem is that the time period during which the B-channel is connected, i.e., a time period during which a circuit path exists on the B-channel is subjected to accounting. Even if facsimile communication is not carried out after repetitive test transmissions, a fee for using the ISDN will be charged.